Method for bleaching paper pulp

ABSTRACT

Various types of paper pulp are bleached in two steps at atmospheric pressure and at a temperature of less than or equal to 100° C. The first step is carried out by bringing the raw pulp into contact with a mixture of peracetic acid and performic acid. The second step is carried out by treating the bleached pulp obtained from the first step, with a solution of sodium hydroxide and hydrogen peroxide. The pulps obtained have a high brightness value and a degree of polymerization close to the raw pulps. The method is ecological and nonpolluting.

[0001] The invention relates to a method for bleaching paper pulpsobtained from lignocellulosic raw materials, in particular plantmaterials of annual plants, agricultural coproducts such as cerealstraw, sugarcane or sweet sorghum bagasses, chips of resinous or leafywood, used paper, and the like.

[0002] The invention aims to provide a method allowing efficient andeconomical bleaching of any type of paper pulps in an aqueous-organicmedium having a water content of less than 15% by weight, so as tolimit, or even eliminate, aqueous polluting discharges and to obtainpaper pulps having a brightness value greater than 70 in only twotreatment steps.

[0003] The invention extends to bleached paper pulps manufactured by themethod and to the paper obtained from said pulps.

[0004] The traditional methods used for bleaching paper pulps useoxidizing agents which are essentially chlorine, oxygen and derivativesthereof.

[0005] The bleaching of paper pulps is carried out by a delignificationoperation which is complementary to that carried out during the heat andchemical treatment to which the plant is subjected, which leads to theraw paper pulp. This operation consists in an oxidative degradation ofthe colored molecules, which are essentially formed of more or lessrecombined lignin fragments, and which form a deposit during cooking onthe paper fibers and of the residual lignin present in the fibers aftercooking.

[0006] Many books and documents describe these operations. Referencewill be made, for example, for further information, to the book ChemicalPulping, J. Gullishen, TAPPI and Paper Oy publisher, p 1 to 145, 2000.(www.tappi.org)

[0007] Chlorine and its derivatives require considerable quantities ofwater, greater than 50 tons of water per ton of pulps. The pollutedwater should then be discharged into rivers.

[0008] The pollution, even with a chemical and biological treatment ofthe effluents before discharge, is considerable with chlorine and sodiumhypochlorite, in particular because of the toxicity of theorganochlorinated compounds generated.

[0009] The use of oxygen, ozone and peroxides of the hydrogen peroxideand peracid type provides a first solution as regards the pollutioncaused by chlorine and its derivatives.

[0010] These technologies are now known and are understood.

[0011] In the same manner, reference may be made, for furtherinformation, to the book Chemical Pulping, J. Gullishen, TAPPI and PaperOy publisher, p 146 to 213, 2000. (www.tappi.org) where thesetechnologies are widely described from very recent references.

[0012] The quantities of water used remain high and these methods allrequire more than two steps of bringing the paper pulp to be bleachedinto contact with chemical reagents.

[0013] Furthermore, it is difficult to control the reactivity of ozoneand of hydrogen peroxide toward cellulose and polysaccharides, whichcauses degradation of the mechanical qualities of the paper pulps thustreated.

[0014] The number of steps of bringing the paper pulps to be bleachedinto contact with the quantity of water used and discharged into theenvironment are as many factors which penalize these technologies fromthe economic and ecological point of view.

[0015] There is another way of manufacturing these bleached paper pulpswhich is radically different from the conventional methods and which iscarried out by oxidative degradation of the lignins which is performedon the initial plant material. These technologies generally useperformic acid and/or peracetic acid generated by direct reaction withhydrogen peroxide, with or without a catalyst.

[0016] The studies by J. Sundquist and colleagues on this subjectappeared in: Papper Och Trä p 88, 2, 1986; and are summarized inChemical Pulping, J. Gullishen, TAPPI and Paper Oy publisher, p 421 to425 (www.tappi.org), show that this type of pulps at mechanicalqualities of a lower quality. Furthermore, when the peracids aredirectly used on the plant, the consumption of hydrogen peroxide and ofperacids is very high, which economically condemns this type of method.

[0017] The present invention proposes to provide a novel method forbleaching paper pulps.

[0018] The main objective of the invention is to provide a novel methodof bleaching which:

[0019] makes it possible to obtain, in only two steps of bringing intocontact with chemical reagents, paper pulps having a brightness valuegreater than 70, which may reach, for paper of higher quality, valuesgreater than 90 (the brightness value is defined on a scale from 0 to100; the value 100 being that for magnesium carbonate: MgCO₃ consideredas the absolute reference in terms of brightness);

[0020] makes it possible to preserve the mechanical properties of theraw pulps without substantially degrading the cellulosic fibers bypreserving in particular a degree of polymerization of said pulps closeto that of the raw pulps before bleaching;

[0021] at an economic cost close to or even less than current industrialmethods since most of the lignin has been removed during the manufactureof the paper pulp;

[0022] is well suited in particular to all types of raw chemical andsemichemical paper pulps.

[0023] Another objective of the invention is to work under conditions ofhydration of the reaction medium which is as low as possible so as toreduce as much as possible the consumption of water and therefore theaqueous discharges generated by the bleaching operations.

[0024] To this end, the method of the invention is characterized in thatit combines two successive treatment steps which are:

[0025] bringing the raw pulp into contact with a mixture of peraceticand performic acid, of acetic and formic acid so that the mixtureobtained has a water content of less than or equal to 15% by weight ofthe mixture;

[0026] treating the pulp obtained after mechanical separation of thereaction medium with a solution of hydrogen peroxide in a slightly basicmedium (8<pH<10).

[0027] The bleaching phase occurs mainly in the first step correspondingto bringing the raw pulp into contact with the mixture of performic andperacetic acid which are indeed potent oxidizing agents capable ofcarrying out the degradation of the residual lignins present,responsible for most of the more or less dark brown color of the rawpulps.

[0028] The experiments showed, surprisingly, that the mixture ofperformic acid and peracetic acid containing little water allowed aparticularly efficient bleaching in a single step without degradation ofthe cellulose fibers.

[0029] This result, which is remarkable per se, should be attributed tothe capacities of these two peracids to act, via reactive speciesgenerated, by rupturing the peracid function, in particular the OH⁺ ion,which degrades, via an electrophilic substitution, the aromatic ringsand the side chains of the lignins, without substantially attacking thecarbonyls and the primary and secondary alcohol functions of the sugars.

[0030] These reactions, which are very selective for lignins, aretherefore without any apparent effect on the cellulose chains, andtherefore on the quality of the bleached paper pulp since, in the methodof the invention, the degree of polymerization of the cellulose fibersis not or is only slightly affected.

[0031] The best results were obtained with a quantity of water limitedto 15% by weight of the reaction medium, which differs considerably fromthe traditional methods, including those using peracids in a bleachingstep, a step in which the quantity of water present in the reactionmedium is considerably larger.

[0032] Under these conditions, the acidity of the medium is buffered bythe presence of acetic acid, which can explain the very low degradationof the cellulose fibers.

[0033] As the bleaching is performed with an excess of peracids, thesolution separated from the bleached pulp is recycled to the peracidproducing reactor and treated with hydrogen peroxide at 50% by weight inorder to bring the peracid concentration to the required value beforebeing sent into the bleaching reactor.

[0034] In this manner, the bleaching agent is in a large excess relativeto the coloring products to be destroyed, which makes it possible tobetter understand the remarkable efficiency of the method.

[0035] The mixture of peracetic acid and performic acid is preferablyprepared by bringing the acetic acid and the formic acid into contactwith hydrogen peroxide at a concentration greater than 50% by weight. Anacetic acid+peracetic acid/formic acid+performic acid ratio of the orderof 9/1 by volume leads to the best results observed experimentally.

[0036] According to a preferred embodiment, the solution of performicacid and acetic acid will circulate countercurrentwise to the stream ofpulps and will be sent, before recirculation, into a reactor forbringing into contact with hydrogen peroxide so as to permanently keepan optimum high concentration in contact with the pulp in circulation.

[0037] Another preferred embodiment will lead to:

[0038] using a concentration of peracids in contact with the raw pulp ofthe order of 20% by weight relative to the dry pulp;

[0039] bringing the raw pulps into contact with the mixture of peracidscirculating countercurrentwise or in a fixed bed reactor withrecirculation of the acids through the pulp;

[0040] regularly reoxidizing through a deviation loop passing into areactor for contact with hydrogen peroxide, the acetic acid and theformic acid to the corresponding peracids.

[0041] The latter operation is made possible by a happy consequence ofthe method which means that the solutions of performic and peraceticacids contain little or no free lignin, which allows their continuousreuse after reoxidation with hydrogen peroxide.

[0042] The contact time between the solution of peracids and the pulp tobe bleached will be preferably between 1 and 3 hours.

[0043] The preferred treatment temperature may be between 60 and 90° C.

[0044] The bleached paper pulp obtained is then deacidified by dryingunder vacuum.

[0045] In a preferred embodiment, the second step is carried out bybringing the deacidified pulp into contact with a basic aqueous solutionat a pH of between 8 and 10, containing 1 to 4% by weight of hydrogenperoxide (calculated relative to the treated dry pulp). The treatmenttemperature will preferably be between 60° C. and 100° C.

[0046] The aim of this operation is to finish the bleaching operation,neutralize the last acidic residues and provide, after washing withdemineralized water, a stable and neutral, and even very slightly basic,pulp.

[0047] The invention extends to the neutral and slightly basic pulpsobtained by this technology which are characterized by a low reductionin the degree of polymerization, less than that observed in conventionalmethods.

[0048] The description which follows, with reference to the appendedFIG. 1, illustrates the method of the invention in an optimumembodiment.

[0049] The synopsis of FIG. 1 illustrates the implementation of theoperations for bleaching the raw pulps. Some operations are known perse.

[0050] The novelty of the method of the invention consists essentiallyin the production of peracids by recycling of the aqueous-organic phasecontaining the organic acids and the residual peracids whose degree ofhydration will be controlled.

[0051] This makes it possible to have as a final result, a mereconsumption of hydrogen peroxide in an operation which is highlysuperior in terms of selectivity and efficiency to a bleaching phaseknown per se directly using hydrogen peroxide which cannot in a firststage reach the remarkable performance of the method of the invention.

[0052] The paper pulps, dried beforehand so as to have a residual watercontent of the order of 20%, are therefore exposed to theaqueous-organic solution containing the peracids at a degree ofhydration by weight of the order of 5 to 8% by weight.

[0053] After a reaction time of the order of 1 to 3 h, theaqueous-organic solution is separated by pressing the pulps whichalready have a brightness value greater than 65.

[0054] The pulps are deacidified in a chamber under vacuum (20 to 30KPa) heated to a temperature between 60° C. and 85° C.

[0055] The organic acids recovered are added to the liquid phaseobtained from the pressing.

[0056] It should be noted that the residual hydrogen peroxide remains,during this operation, in the pulp.

[0057] The solution of organic acids and peracids which is obtained fromthe pressing and from the deacidification operation under vacuum isdehydrated by azeotropic distillation with the aid of a solvent such ascyclohexane (water/cyclohexane azeotrope: boiling point: 69.8° C. atatmospheric pressure, water content 8.5% by weight).

[0058] The aim of this distillation is to maintain the waterconcentration of the organic acids at a value of the order of 4% byweight, before treating them with a solution of hydrogen peroxide at 50%by weight so as to increase the level of peracids in the reactorprovided for this purpose before being sent into the reactor forbleaching in an acidic medium.

[0059] The deacidified pulps are introduced into the second reactor andexposed to a solution of hydrogen peroxide in a slightly basic medium(pH 8 to 10) (2% by weight of hydrogen peroxide at 100% relative to thedry pulp). The duration of the treatment is between 1 and 3 hours for atemperature ranging from 60 to 100° C.

[0060] This operation allows the finishing of the bleaching by a gain ofa few brightness points and makes it possible especially to neutralizethe pulps before washing with demineralized water.

[0061] The neutral pulps separated by pressing are ready for use.

[0062] The washings and the water recovered during dehydration have alow content of organic matter because of the presence, in the bleachingphases, of strong oxidizing agents such as peracids and hydrogenperoxide.

[0063] They can be integrally recycled to the production of pulps which,regardless of the method used, requires a certain quantity of water.

[0064] The method of the invention, in its optimum implementation,therefore does not produce polluting discharges, in a high-performingeconomic context, unlike the current bleaching methods.

[0065] The following examples, which were carried out in a laboratory,illustrate the method of the invention.

EXAMPLE 1

[0066] Bleaching of pulps from wheat straw

[0067] 50 grams of air-dried wheat straw are delignified in an organicacid medium under the conditions described in French patent No. 97 13658of Oct. 30, 1997 (publication No. 2 770 543).

[0068] When the cooking is complete, the pulp obtained (25 grams of drymatter having a kappa value of 30 and a degree of polymerization of 1450) is manually pressed and brought into contact with 500 cm³ of asolution of acetic and formic acid and of peracetic and performic acidshaving an acetic acid+peracetic acid/formic acid+performic acid ratio of9/1 by volume.

[0069] The peracids are prepared by bringing the acetic and formic acidsinto contact with hydrogen peroxide having a concentration of 50% byweight minimum (temperature 60° C., duration 3 hours).

[0070] The concentration of hydrogen peroxide is of the order of 0.35mol per liter of mixture of organic acids.

[0071] The water content of the raw pulps is less than 10% by weight.

[0072] The suspension is kept at 60° C. for 3 hours.

[0073] When the three hours have elapsed, the pulp is filtered, pressedmanually and washed with distilled water.

[0074] The peracids and the residual hydrogen peroxide are assayed.

[0075] The concentration of peracids in the acids recovered is 0.2 molper liter, while this concentration in the starting acids was 0.3 molper liter.

[0076] This hydrogen peroxide is 0.08 mol per liter, while thisconcentration was 0.15 mol per liter in the initial mixture of organicacids and peracids.

[0077] The acids recovered are enriched with hydrogen peroxide so as toserve in a new bleaching operation.

[0078] The pressed pulps, after washing with distilled water, arebrought into contact with a basic solution of hydrogen peroxidecontaining 4% by weight of sodium hydroxide relative to the dry pulp (pHof the order of 10) and 2% by weight of hydrogen peroxide relative tothe dry pulp.

[0079] The liquid/solid ratio is 6/1 by weight.

[0080] After a contact time of two hours at 90° C., the pulps arefiltered, pressed and washed with distilled water and air-dried.

[0081] The kappa value is less than 1.

[0082] The degree of polymerization is 1 350.

[0083] The brightness value is 88.

EXAMPLE NO. 2

[0084] 50 grams of straw are treated as in example No. 1.

[0085] The bleaching of the raw straw pulp (kappa value 30, degree ofpolymerization: 1 450) in acidic medium is carried out as in example No.1.

[0086] After filtration of the pulps and manual pressing in order torecover the maximum amount of acids, peracids and residual hydrogenperoxide, the pulps are degassed under vacuum in a rotary evaporator.

[0087] The pressure is kept at a mean value of 25 Kpa.

[0088] The temperature varies from 60° C. to 80° C. from the beginningto the end of the evaporation phase.

[0089] This makes it possible to recover condensates with differentconcentrations of organic acids and of peracids.

[0090] The hydrogen peroxide is mostly present in the pulps afterevaporation of the acids.

[0091] After this evaporation stage, the pulps are brought into contactwith a sodium hydroxide solution (pH 10) with a liquid/solid ratio of7/1.

[0092] The treatment temperature is 95° C.

[0093] The duration of treatment is 2 hours.

[0094] The kappa value obtained is less than 1.

[0095] The brightness value is 88.

[0096] The degree of polymerization of the bleached pulp obtained is 1350.

EXAMPLE NO. 3

[0097] Bleaching of raw industrial kraft pulps from resinous wood.

[0098] 25 grams of raw industrial kraft pulps from resinous wood (kappavalue of 35, degree of polymerization of 1 500), air-dried, are mixedwith 100 cc of a mixture of acetic, formic, peracetic and performicacids which is prepared as in example No. 1.

[0099] The peracids are prepared by bringing the acetic and formic acidsinto contact with hydrogen peroxide having a concentration of 50% byweight minimum (temperature 60° C., duration 3 hours).

[0100] The concentration of hydrogen peroxide is of the order of 0.35mol per liter of mixture of organic acids.

[0101] The water content of the raw pulps is less than 10% by weight.

[0102] The suspension is kept at 70° C. for 2 hours 30 minutes.

[0103] The pulp is then filtered, manually pressed and washed withdistilled water.

[0104] The peracids and hydrogen peroxide are assayed.

[0105] The concentration of peracids in the acids recovered is 0.18 molper liter, while this concentration in the starting acids was 0.3 molper liter.

[0106] This hydrogen peroxide is 0.07 mol per liter, while thisconcentration was 0.15 mol per liter in the initial mixture of organicacids and peracids.

[0107] The acids recovered are enriched with hydrogen peroxide so as toserve in a new bleaching operation.

[0108] The pressed pulps, after washing with distilled water, arebrought into contact with a basic solution of hydrogen peroxidecontaining 4% by weight of sodium hydroxide relative to the dry pulp (pHof the order of 10) and 2% by weight of hydrogen peroxide relative tothe dry pulp.

[0109] The liquid/solid ratio is 6/1 by weight.

[0110] After a contact time of two hours at 85° C., the pulps arefiltered, pressed and washed with distilled water and air-dried.

[0111] The kappa value is less than 1.

[0112] The degree of polymerization of the bleached pulp obtained is 1350.

[0113] The brightness value is 87.

EXAMPLE NO. 4

[0114] Bleaching of industrial ammonium bisulfite pulps from resinouswood

[0115] 25 grams of industrial ammonium bisulfite pulps from resinouswood (kappa value of 30, degree of polymerization of 1 550), air-dried,are treated as in example No. 3.

[0116] The brightness value of the bleached pulp obtained is 90. Thekappa value is less than 1.

[0117] The degree of polymerization is 1 300.

1- A method for bleaching raw paper pulps with organic peracids,characterized in that it is carried out in two steps at atmosphericpressure and at a temperature of less than or equal to 100° C.: thefirst step is carried out by bringing the raw pulp into contact with amixture of peracetic acid and performic acid so that the mixtureobtained has a water content of less than or equal to 15% by weight; thesecond step is carried out by treating the bleached pulp obtained, aftermechanical separation of the reaction medium obtained at the end of thefirst step, with a solution of sodium hydroxide and hydrogen peroxide.2- The method as claimed in claim 1, characterized in that the peraceticacid and the performic acid are obtained by bringing a mixture of aceticacid and formic acid into contact with hydrogen peroxide having aconcentration greater than 50% by weight. 3- The method as claimed inclaims 1 and 2, characterized in that the acetic acid+peraceticacid/formic acid+performic acid ratio is of the order of 9/1 by volume.4- The method as claimed in claims 1, 2 and 3, characterized in that thequantity of peracids at the beginning of the first step is greater thanor equal to 20% by weight. 5- The method as claimed in claims 1, 2, 3and 4, characterized in that the raw pulps are brought into contact withthe mixture of peracids countercurrentwise with recirculation of theperacids. 6- The method as claimed in claims 1, 2, 3, 4 and 5,characterized in that the solution of peracids after the first bleachingstep is enriched with peracids so as to bring the concentration ofperacids to the chosen value. 7- The method as claimed in claims 1, 2,3, 4, 5 and 6, characterized in that the pulps treated during the firststep are separated from the peracids by pressing. 8- The method asclaimed in claims 1, 2, 3, 4, 5, 6 and 7, characterized in that thepressed pulps are deacidified by drying under vacuum, leaving residualhydrogen peroxide in the pulp. 9- The method as claimed in claims 1, 2,3, 4, 5, 6, 7 and 8, characterized in that the contact time between theperacids and the raw pulp is between 1 hour and 3 hours. 10- The methodas claimed in claims 1, 2, 3, 4, 5, 6, 7, 8 and 9, characterized in thatthe treatment temperature is between 60° C. and 90° C. 11- The method asclaimed in claims 1, 2, 3, 4, 5, 6, 7, 8, 9 and 10, characterized inthat the second step is carried out by bringing the deacidified pulpinto contact with a basic aqueous solution (pH between 8 and 10)containing 1 to 4% by weight of hydrogen peroxide (calculated relativeto the dry pulp). 12- The method as claimed in claims 1, 2, 3, 4, 5, 6,7, 8, 9, 10 and 11, characterized in that the contact time between thepulp and the hydrogen peroxide in a basic medium is between 1 h and 3 h.13- The method as claimed in claims 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11and 12, characterized in that the pulps obtained during the second stepare, after pressing, washed with demineralized water. 14- The method asclaimed in claims 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12 and 13,characterized in that the water introduced, by the raw pulp, on the onehand, and the hydrogen peroxide, on the other hand, is extracted fromthe cycle by distillation. 15- A paper pulp, obtained according to themethod of claims 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13 and 14 havinga brightness value greater than
 70. 16- A paper, manufactured using apulp in accordance with claim 15.